In the field of image processing, it is often the case that an image must be resized for a desired application. For example, the pixels per inch (or dpi dots per inch) of many display devices are fixed. A particular printer may have a resolution of 250 dpi, for example. In order to print an image consisting of 500 by 750 pixels to a 4 by 6 inch print on such a printer, the image must be resized by a factor of 2. In other words, the values of the image at locations between the original samples must be determined. This process of determining the value of an image signal at locations which are not coincident with the original samples of the image is called interpolation. The factor of interpolation, N, refers to the ratio of the sampling rate of the output image to the sampling rate of the input image. In the previous example, the interpolation factor N=2.
The process of interpolation produces an image of a desired number of rows and columns of pixels. However, the interpolation process involves “guessing” the signal value at many locations. As a result the interpolated image is generally not as high quality as an image originally captured at the desired resolution. For example, an image captured at 500×750 pixels and then interpolated to 1000×1500 pixels will generally appear softer and of lower quality than an image resulting from capturing that same scene at 1000×1500 pixels originally.
Because of the generally lower quality of interpolated images, it would be a desirable and valuable goal for an image processing system to have knowledge of whether an image had been interpolated. More precisely, it would be valuable for an image processing system to be able to distinguish a non-interpolated image from an interpolated image. Additionally, it would be desirable for the image processing system to have knowledge of the factor of interpolation N.
Consequently, it would be desirable to determine directly from the data contained within a digital image (the pixel locations and values) whether an image has been interpolated. It would further be desirable to determine the factor of interpolation directly from the data contained within the digital image.